The invention relates to “active implantable medical devices” as defined by the 90/395/CEE directive of Jun. 20, 1990 of the European community counsel. This definition notably includes the devices that continuously monitor the cardiac activity and deliver if necessary to the heart electrical pulses of stimulation, cardiac resynchronization, cardioversion and/or defibrillation in case of a rhythm disorder detected by the device. It also includes the neurological devices, the cochlear implants, etc., as well as the device for pH measurement or other intracorporeal parameters.
These devices include a generator consisting of a metal housing, usually made of titanium, on which a connector head is mounted. The connector is provided with housings for mechanically and electrically connecting one or more leads to the generator housing, the leads having at their distal end various electrodes of sensing, pacing and defibrillation. The connection of the connector to various electronic circuits involves the realization of several electrical feedthroughs, between connectors assembled on the upper surface of the housing (outer side), and the interior volume of the housing where these circuits are (inner side).
Besides the connection pins on the connector head, other feedthroughs can also be provided, for example to ensure a connection with a surface electrode placed on the outside of the housing, or at the sensor integrated with a lead of the device. These feedthroughs can also be found in sub-components of medical devices such as batteries and capacitors. Such a feedthrough is for example described in EP2377573 A1 (Sorin CRM SAS).
The technique described in the EP2377573 A1 document is to superficially oxidize the outer side of the titanium housing or to deposit an insulating layer (e.g. silicon dioxide) on the outer side, and, on the inner side, to dig into the wall throughout its thickness, so as to form a contour groove defining a closed area or “islet” dedicated to the electrical conduction. This islet, created in the mass of the wall of the housing, is physically and therefore electrically isolated from the rest of the body of the housing.
An electrical connection is then performed on each side of the islet by providing on each side thereof a contact pad on which for example a connection wire to a terminal of the electronic circuit, or a connection pin of the connector, is welded. The presence on the outer side of the insulating layer, which is not inground, ensures a perfect hermeticity of the feedthrough and prevents penetration of fluid inside the housing. This layer also has the advantage of being biocompatible, biostable and resistant to corrosion.
The method can also be developed without significant additional cost, in so far as it uses only proven conventional techniques. However, this technique leaves some mechanical fragility. Indeed, after excavation of the peripheral groove surrounding the islet, the latter is completely detached from the rest of the wall of the housing (which is precisely essential to ensure the electrical insulation of the feedthrough) and is only connected to this wall by the thin superficial layer that forms an oxide “membrane” or “diaphragm” whose thickness is typically 10 to 15 microns (for a wall thickness of about 300 microns).
This residual fragility, which is intrinsic to the thinness of the oxide layer, is further increased by the rather average tolerances achieved downhole during the digging of the groove. This can locally lead to the appearance of cracks or other micro-defects, precisely near the thin oxide layer which holds the islet and wherein stress biasing the structure may be concentrated, for example because of the wires or pins welded onto the central islet.
Certain proposed embodiments of the present invention provide a solution of mechanical reinforcement of this known structure to make it more robust and tolerant to the presence of micro-defects.
Another inherent drawback in the structure described above is that it only allows to passively convey (ohmic, purely resistive conduction) an electrical signal between the inside and outside of the housing. Therefore, to provide a filter (series or parallel) to the feedthrough, it is necessary to provide an external capacitive disc carried on the inner face of the feedthrough, to which the disc is mechanically and electrically connected, for example by a conductive adhesive. This is a relatively expensive step from the industrial viewpoint since it requires numerous sub-steps that can also cause reliability problems.
Certain proposed embodiments of the present invention provide various collateral advantages, including the ability to integrate into the known structure, improved according to the invention, additional functional elements, such as an RF antenna (for the purpose of RF telemetry), electrodes, sensor elements etc. Indeed, the RF antennas, for example, are currently made from a metal wire outside the housing, which is connected to the internal electronic circuitry through a dedicated feedthrough which is then overmolded in a biocompatible plastic matrix. Again, the connection and industrialization are complex and induce an overall volume significantly higher than that of devices without RF antenna.